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Cormorant indices for England

This page hosts the Official Statistic 'Provisional Cormorant population indices for England 2022', published on 14 July 2022.

Official Statistic description

  • This release includes provisional indices of Cormorant population in England, showing change in population between 1987/88 and 2021/22. The statistics are based on data gathered through the BTO/RSPB/JNCC Wetland Bird Survey (WeBS), which are published annually in the Spring.
  • Cormorant indices up to the year 2021/22 are being released early as provisional to enable their use for operational purposes before the main WeBS Official Statistics release in Spring 2023. These provisional indices are based on the vast majority of England Cormorant data that are expected to be included in the subsequent WeBS Official Statistics release, and are therefore expected to be a good indication of the the final results.
  • Technical details of the WeBS method are set out in a separate document available on the BTO website
  • The Wetland Bird Survey (WeBS) is a partnership jointly funded by the British Trust for Ornithology (BTO), Royal Society for the Protection of Birds (RSPB) and JNCC, with fieldwork conducted by volunteers and previous support from the Wildfowl and Wetlands Trust (WWT).

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Scope of statistics

  • This release covers Cormorant data from England only.
  • Statistics include unsmoothed and smoothed annual population indices from 1987/88 to 2021/22.
  • 25-year and 10-year trends are reported based on the smoothed indices.

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Who was involved and key contact point

  • These statistics were produced by BTO with Quality Assurance carried out by BTO and JNCC.
  • The data are published as a JNCC Official Statistic. If you have any queries, please get in touch.

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Relation to any other National/Official Statistics

  • An updated version of these statistics will be included in the Spring 2023 Official Statistics release 'Statistics on Waterbirds in the UK' (the Spring 2023 update will include data up to 2022).
  • ‘Statistics on Waterbirds in the UK’ feeds into the National Statistics Compendium UK Biodiversity Indicators, and country-level indicators (e.g. for Scotland and for England) and will feed into the Outcome Indicator Framework for the 25 Year Environment Plan in England.
  • These statistics forms part of a suite of statistics produced through partnership monitoring schemes as part of JNCC’s terrestrial evidence programme. 

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Confidence in results and caveats

  • WeBS volunteers follow standardised, peer-reviewed methods to ensure results are scientifically robust. The submitted data undergo a combination of automated and manual validation and verification processes.
  • Trends are produced using Generalized Additive Models based on changes in smoothed population indices – this reduces the risk that short-term fluctuations obscure long-term trends. Missing values are accounted for using the Underhill indexing method (Underhill L.G. & Prys-Jones, R. 1994. Index numbers for waterbird populations. I. Review and methodology. Journal of Applied Ecology, 31, 463–480).
  • Although the statistical significance of trends is not currently assessed, the statistical approaches used to generate and compare indices are routinely used in peer-reviewed publications.
  • Data from the 2021/22 recording season used in these provisional indices amounted to 92% of the number of sites that were used in the calculation of the 2019/20 statistics. This current analysis included all of the top 16 sites for Cormorant in England (i.e. sites with the highest 5-year mean peak as of 2019/20), and 44 of the top 45 sites. Therefore, whilst some additional data are expected to be submitted, the indices for Cormorants in England are not expected to change much between July 2022 and spring 2023.
  • It should be noted that the indices for penultimate year 2020/21 (and consequently exact values in the trend analysis) should be viewed with caution as they were heavily impacted by reduced survey coverage during Covid-19 restrictions. Whilst the models used account for missing data (as opposed to assuming an unsurveyed site had no Cormorants), the more data that are missing the greater the risk of inaccuracy and potential bias in results. Investigation of the dataset suggests that the penultimate data point could be negatively biased. However, the year 2021/22 had much higher survey coverage (even taking onto account the fact that some data are still to be submitted at this stage) and this year showed even lower index values, suggesting that there has been a genuine decline in the latest year. Further detail is provided in the spreadsheet of results.

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Summary of results

Graph showing the cormorant indices for England between 1987 to 1988 and 2021 to 2022. The data are provided in the accompanying spreadsheet

  • The Cormorant population in England increased since monitoring began in the late 1980s.
  • However their population indices have shown a recent decline.
  • The 25-year trend (based on smoothed indices 1995/96 to 2020/21) shows an increase of 50%, and the 10-year trend (based on smoothed indices 1995/96 to 2020/21) shows a smaller increase of 23%.
  • Note, for statistical reasons it is customary to truncate the final year when reporting smoothed trends, so whilst data from 2021/22 have been used in creating the smoothed index values, the trend period assessed and reported is until 2020/21.
  • The data are provided in a spreadsheet and are available under Open Government Licence (attribution statement: Contains Wetland Bird Survey (WeBS) data © copyright and database right 2022. WeBS is a partnership jointly funded by the BTO, RSPB and JNCC with fieldwork conducted by volunteers and previous support from WWT)

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